Disclosed herein is a digital radiographic detection system and, more specifically, a quantum dot digital radiographic detection system. Digital radiography (“DR” or “DX”) is a form of X-ray imaging, where a semiconductor visible light detection device (e.g. digital X-ray sensors or imagers) is used instead of traditional photographic film. The semiconductor visible light detection device is used to record the X-ray image and make it available as a digital file that can be presented for interpretation and saved as part of a patient's medical record. U.S. Pat. No. 7,294,847 to Imai, U.S. Pat. No. 7,250,608 to Ozeki, and U.S. Pat. No. 5,017,782 to Nelson describe examples of digital radiographic detection devices (also referred to as “radiographic detectors”) and related technology and are herein incorporated by reference. Advantages of digital radiography over traditional photographic film include, but are not limited to, the fact that digital radiography has the ability to digitally transfer images, the ability to digitally save images, the ability to digitally enhance images (e.g. the ability to apply special image processing techniques that enhance overall display of the image), the ability to use images that might otherwise have been insufficient (e.g. a wider dynamic range makes digital radiography more forgiving for over- and under-exposure), the ability to immediately have an image available for preview (e.g. time efficiency through bypassing chemical processing), the ability to use less radiation to produce an image of similar contrast to conventional radiography, and the ability to reduce costs (e.g. costs associated with processing film, managing film, and storing film).
Conventional digital radiographic detection devices (also referred to as “silicon-based light detection devices”) currently use digital image capture technologies such as CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) image sensors (also referred to as “semiconductor visible light detectors” or “imagers”) as the underlying semiconductor technologies. Both CCD and CMOS image sensors are silicon-based image sensors that require overlying scintillation layers for indirect conversion of X-rays into visible light. Both CCD and CMOS image sensors use light detectors to read the overlying scintillation layer. Both types of image sensors convert light into an electric charge and process it into electronic signals. In a CCD image sensor, every pixel's charge is transferred through a very limited number of output nodes (often just one output node) to be converted to voltage, buffered, and sent off-chip as an analog signal. Because all of the pixels in the CCD sensor can be devoted to light capture, the CCD sensor has a high output uniformity (which generally results in better image quality). In a CMOS image sensor, each pixel has its own charge-to-voltage conversion so the CMOS image sensor has lower output uniformity than the output of the CCD image sensor. On the other hand, the CMOS image sensor can be built to require less off-chip circuitry for basic operation. The CMOS image sensor also includes additional functions such as amplifiers, noise-correction, and digitization circuits so that the CMOS image sensor chip outputs digital bits.
Conventional silicon-based image sensors (including CCD and CMOS) have been used for indirect conversion of ionizing X-radiation into visible images for medical and dental use. There are, however, inherent physical drawbacks to the use of CCD and CMOS sensors for X-radiography including, but not limited to the requirement of relatively thick scintillation layers, the requirement that detectors must be embedded within the physical body of the silicon device, the requirement of large individual detector sizes, low detector efficiency for capturing generated photons, low active sensor detection area/total detector size ratio, the inability to optimize peak sensor optical sensitivity to the scintillation chemistry, and the narrow practical dynamic range between over- and under-exposure by the practitioner. These limitations result in a blurred image, low sensor image contrast, and a narrow dynamic range. A wide variety of techniques, including unique physical designs of the scintillation layer and software compensations, are required to minimize these limitations.
From a practitioner's perspective, direct digital radiographic detection devices that use CCD and CMOS image sensors have diagnostic qualities that are very poor as compared to direct digital radiographic detection devices that use traditional film. Digital radiographic detection devices that use CCD and CMOS image sensors have poor edge definition in the native image, poor contrast levels in the native image, very narrow dynamic range between over- and under-exposed images, and most of the photons generated by the scintillation layer (over 95%) are simply not detected. Without significant software enhancement CCD and CMOS images would not be diagnostic. The limitations are inherent to how CCD and CMOS image sensors function.
A quantum dot (fluorescent semiconductor nanocrystal) is a semiconductor whose excitations are confined in all three spatial dimensions. As a result, the quantum dots have properties that are between those of bulk semiconductors and those of discrete molecules. Simplistically, quantum dot detectors are semiconductors whose conducting characteristics are closely related to the size and shape of the individual crystal. Generally, the smaller the size of the crystal, the larger the band gap, and the greater the difference in energy between the highest valence band and the lowest conduction band becomes, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. One of main advantages in using quantum dots is that because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material and fine tune the peak sensitivity to the frequency being detected.